Lead-acid batteries have conventionally been used in various industrial fields, for example, as car batteries and back-up power sources. With respect to lead-acid batteries used in automotive applications, it is required to reduce the amount of self-discharge and the amount of loss of water contained in electrolyte (hereinafter referred to as water loss) for decreasing the number of man-hours needed for repair and maintenance. To meet such requirements, grids used in positive and negative electrodes are made of a lead-calcium alloy, which is free from antimony that increases the amounts of self-discharge and water loss.
Among them, expanded grids obtained by making slits in a rolled sheet of a lead-calcium alloy and expanding the slits have an advantage of high productivity. Also, the addition of tin to a lead-calcium alloy provides high mechanical strength and corrosion resistance required of lead-acid battery grids. Thus, expanded grids made of a Pb—Ca—Sn alloy are widely used.
FIG. 1 shows a production method of a lead-calcium alloy sheet used in a conventional expanded grid. A slab 1, which is a plate-shaped base material and obtained by continuous casting of a lead-calcium alloy, is gradually rolled by a plurality of pairs of rollers 2. At this time, the distance between each pair of rollers in FIG. 1 gradually decreases as the thickness of the slab 1 decreases. That is, the plurality of pairs of rollers are arranged such that the radius (rn+1) of the n+1st pair of rollers 2 is greater than the radius (rn) of the nth pair of rollers 2. Also, the center distance between adjoining nth pair of rollers 2 and n+1st pair of rollers 2 is constant. The slab 1 is eventually rolled to a desired thickness, to obtain an alloy sheet 3.
It should be noted that other than the method of FIG. 1, rollers 4 may be arranged such that the center distance between each pair of rollers 4 gradually decreases as the thickness of the slab 1 decreases, while the radius (r) of the rollers 4 is made constant, as illustrated in FIG. 2.
Thereafter, slits are cut in the alloy sheet 3, and the slits are expanded to obtain an expanded grid having meshes. An active material paste is filled into the meshes of the expanded grid, and the resultant grid is cut to obtain an electrode plate for a lead-acid battery.
As described above, when electrode plates including an expanded grid made of a lead-calcium alloy are used as positive and negative electrodes, there is an advantage that the amounts of self-discharge and water loss of the resultant lead-acid battery are small, in comparison with the cases of including a positive electrode grid made of a lead-antimony alloy. However, there is also a drawback of poor cycle life characteristics in repeating charging and discharging.
As a method for improving the cycle life characteristics, Japanese Laid-Open Patent Publication No. Sho 61-200670 proposes placing a lead alloy sheet containing one or both of tin and antimony on a base material sheet of a lead-calcium alloy, and rolling these two sheets for integration to obtain a composite sheet. Since tin or antimony contained in the composite sheet has the effect of improving the adhesion of the positive electrode active material to the positive electrode grid, the cycle life characteristics are improved.
In improving the adhesion of the positive electrode active material to the positive electrode grid containing tin or antimony, it is important that the base material sheet and the lead alloy sheet have good adhesion. In the next step, the composite sheet is expanded in an expanding process and therefore undergoes plastic deformation. In this step, if the adhesion of the lead alloy sheet to the base material sheet is not good, tiny cracks tend to occur between the base material sheet and the lead alloy sheet. The occurrence of the cracks significantly impairs the adhesion of the positive electrode active material to the positive electrode grid, resulting in a decrease in cycle life characteristics.
As a method for improving the adhesion of the lead alloy sheet to the base material sheet, Japanese Laid-Open Patent Publication No. Hei 5-13084, for example, proposes making the temperature difference between the base material slab and the lead alloy foil attached to this slab 150° C. or less. Also, as a method of obtaining such temperature difference, there is a proposal of cooling the slab surface with water.
In this way, by regulating the temperature difference between the slab and the lead alloy foil, it is possible to suppress, to some extent, the separation of the lead alloy foil which occurs when the composite sheet consisting of the slab and the lead alloy foil is bent. However, completely preventing the occurrence of such separation is still difficult. Further, even if the separation of the lead alloy foil is not found by the visual inspection of the composite sheet, a lead-acid battery including this composite sheet as the positive electrode grid may not exhibit expected cycle life characteristics. In this case, it is considered that there is such minute separation between the base material sheet and the lead alloy sheet that cannot be found by visual inspection.
In view of the above problems, it is therefore an object of the present invention to provide a method for producing a lead-acid battery grid formed from a composite sheet, which has excellent adhesion between a base material sheet of a lead-calcium alloy and lead alloy foil containing a component effective for improving cycle life characteristics. It is another object to provide a lead-acid battery having good cycle life characteristics by using this composite sheet, which has been subjected to an expanding process, as a positive electrode grid.